Liquid leaks from a petroleum pipeline, petroleum tanks and the like can become a cause of various accidents, for example environmental pollution, fire or explosion. Therefore, it is established by law in certain countries that equipment be installed for detecting liquid leaks from petroleum pipelines and tanks. Also, it is desirable to install such leak detecting equipment on gas holding tanks, storage tanks and pipings used in chemical plants which contain dangerous liquids such as fuels, solvents or poisonous liquids.
In order to detect liquid leaks, many proposals have previously been made. Of the conventional means, those utilizing light and electricity are listed below:
1. Light Attenuation Method. Attenuation of light, caused by the intrusion of liquid leak into a detecting device (fiber-optic cable), is measured to detect a leak.
2. TDR Method. Capacitance change of a leak detecting cable consisting of two parallel conductors separated by a porous insulation, caused by the impregnation of the liquid to be detected, is measured by a TDR to indicate liquid leaks. (U.S. Pat. No. 3,981,181).
3. Short Circuit Method. A short circuit occurring in a cable consisting of two conductors separated by a soluble material when the soluble material is dissolved by the liquid leak, sounds an alarm. (Canadian Patent No. 775,758).
4. Float Method. A float having two electrodes is put on the surface of water. The electrical resistance between the electrodes is increased to indicate an oil leak when oil spreads on the surface of the water.
The method mentioned above, depending upon the application, have certain advantages and in other applications certain disadvantages. For example, Method 1 and 2 above are likely to be affected by moisture or steam. Method 3 above is less reliable and may cause a danger of sparking. Method 4 is easily affected by the movement of the water surface, and adjustments of the electrodes are delicate and unstable.
U.S. Pat. No. 3,412,358 provides an electrical resistance material, used for example as a heating element when voltage is applied, comprising (1) at least one resistive substance such as carbon black, (2) an insulating material such as oils, and (3) a nonconductive carrier such as polyethylene. These materials are mixed together and extruded into foils, plates or the like, and when voltage is applied across the foil, heat is generated. There is no suggestion in this reference of use of this material in leak detection nor that the conductivity of this material will change when in contact with different liquids. There is no suggestion in this reference of using porous PTFE. The polymeric materials cited in the reference all appear to be conventional, melt extrudable, polymers which would produce nonporous, filled articles upon extrusion.
U.S. Pat. No. 3,981,181 shows porous PTFE used in a leak detecting probe. Therein, PTFE insulated wires are laid along a pipe, for example, and an electrical pulse is sent down the wires. The characteristic impedance of that cable is monitored and, when a leak occurs, its location is detected by a measured change in this characteristic impedance. This reference contains no conductive filler nor any suggestion to provide such as conductive filler.
U.S. Pat. No. 3,979,667 discloses an indicator used to measure the moisture content of soil. This reference shows liquid detecting probe means. The probe of this reference contains none of the key elements of the instant invention nor any suggestion of them.